Semiconductor-type pressure transducer

ABSTRACT

A semiconductor-type pressure transducer is disclosed in which the pressure change is detected as a resistance change by use of a bridge circuit including at least a gauge resistor changing with an external force. Each gauge resistor is made of a PN junction of a semiconductor. The pressure transducer further comprises an amplification factor compensator for cancelling the effect of the temperature change of the gauge resistors making up the bridge circuit on the amplification factor of the amplification circuit for amplifying the output of the bridge circuit.

The present invention relates to a semiconductor-type pressuretransducer, and more in particular to a semiconductor-type pressuretransducer comprising means for compensating for changes in sensorsensitivity with temperature variation.

A semiconductor-type pressure transducer having compensation for changesin sensitivity with temperature variation of the pressure transducer isknown, as disclosed, for example, in the Monolithic Pressure TransducersCatalog issued by National Semiconductor in 1979, in which a thindiaphragm section is formed at the central part of a silicon singlecrystal plate, four gauge resistors are formed by impurity diffusion onthe surface of the diaphragm and connected to make up a bridge circuit,and a temperature-compensating transistor circuit is integrally formedon the surface of the diaphragm or the surrounding thick portion of thesilicon single crystal plate and connected in series between the bridgecircuit and a power supply.

This circuit is capable of reducing the sensitivity changes due totemperature variation of the bridge circuit. However, the transducer isgenerally used in combination with an amplifier for amplifying theoutput of the bridge circuit and the amplification factor of theamplifier also changes with temperature variation. As a result, anaccurate output is unobtainable solely by a temperature compensationcircuit arranged for compensating for the sensitivity change of thebridge circuit alone.

The object of the present invention is to provide a semiconductor-typepressure transducer which is capable of reducing the change of theamplification factor of the amplifier for amplifying the output of abridge circuit of the transducer due to temperature variation thereof.

According to the present invention, a semiconductor-type pressuretransducer is provided with amplification factor compensation means forcancelling the effects of the temperature changes of gauge resistorsmaking up a bridge circuit have on the amplification factor of anamplifier for amplifying the output of the bridge circuit.

The objects, features and advantageous effects of the invention will bewell understood from the following description of the embodiments of theinvention in conjunction with the accompanying drawings, in which:

FIG. 1 is a circuit diagram showing an embodiment of the presentinvention;

FIG. 2 is a diagram showing the characteristic for explaining thepresent invention;

FIG. 3 shows a material circuit arrangement of the embodiment shown inFIG. 1;

FIG. 4 shows an output characteristic of the circuit of FIG. 3; and

FIG. 5 is a diagram showing another embodiment of the present invention.

An embodiment of the present invention shown in the drawings will bedescribed in detail.

Reference characters G₁ to G₅ designate gauge resistors made by impuritydiffusion in the surface of a silicon diaphragm. Of these gaugeresistors, the gage resistors G₁ to G₄ make up a Wheatstone bridgeconnected in such a manner that the gauge resistors G₁ and G₂ areconnected to the sides of the bridge circuit, respectively, diagonal tothe sides to which the gauge resistors G₃ and G₄ are connected,respectively.

A drive power circuit connected to the power terminal P of the bridgeincludes a current source I_(b) for supplying a current controlled tochange in proportion to the temperature of the bridge, a resistor R₁₁ inseries with the current source I_(b), a gauge resistor G₅, and anoperational amplifier OP₃ for amplifying the voltage at the junctionpoint a of the current source I_(b) and the resistor R₁₁ to be suppliedto the power terminal P of the bridge.

The gauge resistors G₁ to G₄ are set so that the values of G₁, G₂, G₃and G₄ are equal to a predetermined value G when no pressure is appliedto the diaphragm.

The output terminal Q₁ of the bridge is connected through a resistor R₈to the positive terminal of the operational amplifier OP₂, and thebridge output terminal Q₂ is connected to the negative terminal of theoperational amplifier OP₂ through the resistor R₇.

R₉ designates a feedback resistor of the operational amplifier OP₂, andR₁₀ a dividing resistor.

The resistors R₇ to R₁₁ are substantially insensitive to temperaturechanges.

The resistance values of the resistors R₇ and R₉ are equal to those ofthe resistors R₁₁ and R₁₀, respectively.

The voltage V₄ at the power terminal P of the bridge is regulated to beequal to the voltage V₃ at point a by a voltage follower circuit made upof the operational amplifier OP₃.

When pressure is exerted on the diaphragm carrying the bridge, thebalance of the bridge is broken so that the potential difference V₁ -V₂between the output terminals Q₁ and Q₂ changes according to the appliedpressure.

A balanced-input type amplifier circuit including the operationalamplifier OP₂ and the resistors R₇ to R₁₀ is for amplifying the outputvoltage V₁ -V₂ of the bridge into an output voltage value V₀.

Now, assume that the temperature around the bridge is changed whilemaintaining constant the voltage V₄ of the power terminal P and thepressure applied to the diaphragm. The output of the bridge V₁ -V₂changes with temperature variation along the curve 9 depicted in FIG. 2.

In the curve 9, it is assumed that V₀ =V₁ -V₂. The sensitivity changerate δ of the bridge is expressed by ##EQU1## where V₀ (T) is the bridgeoutput at temperature T and V₀ (20) the bridge output at 20° C.(reference temperature).

From this, it is seen that the bridge sensitivity change withtemperature variation may be compensated for by controlling the voltageV₄ applied to the bridge power terminal P so as to follow the dashedcurve 10 in FIG. 2 which is inclined oppositely to the curve 9 withtemperature.

For this purpose, the current source I_(b) is so constructed that theoutput current thereof changes in proportion to temperature.

The problem of this circuit, however, is that the amplification factorof the amplification circuit including the operational amplifier OP₂changes with variation of the resistance values of the gauge resistorsG₁ to G₄, which in turn change with temperature variation, therebypreventing optimum compensation.

This fact will be explained semi-quantitatively. The constants of anamplifier circuit of the pressure transducer are generally determined tofulfill the condition G<<R₈ +R₁₀. The relation between the output V₀ ofthe amplifier circuit and the bridge output V₁ -V₂ is thus expressed bythe proximation ##EQU2##

The amplification factor K of the amplifier circuit is thus given by##EQU3## where G₀ is the gauge resistance at the reference temperature,α_(G) (T) the change of the gage resistance with temperature variation,K₀ the amplification factor of an ideal balanced-input amplificationcircuit (K₀ =R₉ /R₇), and γ the ratio of the resistance 1/2G₀ to theinput resistor R₇ (γ=1/2G₀ /R₇).

According as the gauge resistance value G₀ increases as compared withthe input resistor R₇, the effective amplification factor K decreaseswhile the change of the effective amplification factor K withtemperature variation follows almost an opposite trend to the change ofthe gauge resistance G with temperature variation.

As a result, even if the current source I_(b) were provided with atemperature characteristic so that the voltage V₄ applied to the powerterminal P follows the curve 10 of FIG. 2, the second term in thebrackets { } of equation (3) should cause insufficient compensation dueto the decrease of K on high temperature side. This insufficientcompensation is derived from the temperature characteristic of the gaugeresistor G, and may be compensated for by a resistor formed by the sameprocess as the gauge resistor G. The resistor G₅ has such a function.

The condition that the resistor G₅ is required to have will beexplained.

In consideration of the temperature characteristic of the transducersensitivity, the equation (2) may be rewritten as follows. ##EQU4##where α_(S) (T) is the change of the output voltage of the pressuretransducer bridge with temperature shown in the curve 9 of FIG. 2, andv₀ the bridge output per unit drive voltage of the bridge under apredetermine pressure.

The potential V₃, on the other hand, is given as ##EQU5## where G₅₀ isthe resistance value of G₅ at the reference temperature, I_(b0) theoutput current of the current source at the reference temperature, β_(S)(T) the temperaure-dependent term of the current source I_(b) with asign opposite to α_(S) in equation (4) as shown in the curve 11 of FIG.2. This term is in fact set to 1/(1+α_(S))-1. For facilitating theunderstanding, however, it is assumed that 1/1+α_(S) ≅1-α_(S).

Therefore, if the following equation (6) is satisfied,

    G.sub.50 /R.sub.11 ≅γ                      (6)

a relation (7) is obtained by applying the equation (5) to the equation(4) and ignoring the temperature-dependent terms of high order

    V.sub.0 ≅K.sub.0 v.sub.0 R.sub.11 I.sub.b0       (7)

This relation (7) shows that the voltage V₀ is independent oftemperature.

The foregoing description is summarized as follows:

(1) The temperature characteristic of the current source I_(b) is madeequal to the characteristic shown by curve 11 in FIG. 2.

(2) The ratio between the resistor R₁₁ and the gage G₅ is selected tosatisfy

    G.sub.5 /R.sub.11 ≅1/2G/R.sub.7                  (8)

When the dimensions of the components are set as shown above, thevariation of the amplification factor of the amplification circuitdependent on the temperature change of the gauge resistor can becancelled.

According to present embodiment, in the case the bridge circuit, theamplification circuit and the temperature compensation circuit areformed on the same chip, it is desired that the resistor G₅ and thegauge resistors G₁ to G₄ are made by the same process so as to providethe same temperature characteristic and that the gauge resistor G₅ isdisposed near to the gauge resistors G₁ to G₄ so that the former issubject to the same temperature condition as the latter.

The resistor G₅, however, may be made as a discrete resistor so long asit has substantially the same temperature characteristic as the gaugeresistors G₁ to G₄ and is subjected substantially to the sametemperature conditions as the latter.

FIG. 3 shows a material circuit arrangement of the current source I_(b).A resistor 20 with an end thereof connected to a power supply V_(CC) hasthe other end thereof connected to the emitter of the transistor Q₆, thecollector of which is connected to the junction point a.

The base of the transistor Q₆ is connected to the base of the transistorQ₄ on the one hand and to the emitter of transistor Q₅ on the otherhand.

The emitter of the transistor Q₄ is connected to an end of the resistorR₁₉ the other end of which is connected to the power supply V_(CC). Thecollector of the transistor Q₄ is connected to the base of transistor Q₅and the collector of transistor Q₃. The collector of the transistor Q₅is connected to the earth GND.

The emitter of the transistor Q₃ is connected to the earth GND throughthe resistor R₁₈, and the base thereof to the emitter of the transistorQ₂ and the base of the transistor Q₁.

The collector of the transistor Q₂ is connected to the power supplyV_(CC), and the base thereof to an end of the resistor R₁₆ the other endof which is connected to the power supply V_(CC).

The emitter of the transistor Q₁ is connected to the earth GND throughthe resistor R₁₇.

The characteristic of the circuit connected in this way is set byadjusting the area ratio of the emitters of transistors Q₄ and Q₆ andthe resistance values of the resistors R₁₉ and R₂₀ to fit the curve 11of FIG. 2.

FIG. 4 shows the sensitivity change rates of the output V₀ of theamplification circuit in the presence and absence of the gauge resistorG₅ for comparison therebetween.

The curve 12 shows the change rate in the absence of the gauge resistorG₅, and the curve 13 the change rate in the presence of gauge resistorG₅.

In the case where the gauge resistor G₅ is omitted, the high-ordertemperature dependent term of γα_(G) (T)/1+γ in equation (4) fails to becompensated, and therefore, the change rate is greater than that in thepresence of the gauge resistor G₅.

Another embodiment of the present invention is shown in FIG. 5.

If the resistors R₇ and R₈ shown in FIG. 1 are replaced by impuritydiffusion resistors G₆ and G₇ formed in the same process as the gaugeresistors G₁ to G₄, the operational amplifier OP₃, and the resistors R₁₁and G₅ are eliminated as shown in FIG. 5, so that a voltage can besupplied to the bridge directly from the current source for generatingan output current proportional to temperature change.

This is because the function of the gauge resistor G₅ to compensate forvariation of the amplification factor is given by the resistor R₇ if theresistor R₇ has the same temperature characteristic as the gaugeregistors G₁ to G₄, as seen from equation (8).

It will be understood from the foregoing description that according tothe present invention, there is provided a semiconductor-type pressuretransducer comprising means for eliminating the effect that thetemperature characteristic of the gauge resistors making up the bridgecircuit might otherwise have on the amplification factor of theamplification circuit for the bridge output, thereby producing anaccurate output of the pressure transducer.

We claim:
 1. A semiconductor-type pressure transducer comprising abridge circuit including a plurality of gauge resistors at least one ofwhich has a resistance which varies with an external force, anamplification circuit connected to an output of said bridge circuit foramplifying the output of said bridge circuit, and a drive power circuitconnected to a power source terminal of said bridge circuit andconnected between first and second potential voltage levels forsupplying a drive voltage to said bridge circuit, wherein said drivepower circuit includes a resistor coupled in series between said firstand second potential voltage levels and having the same temperaturecharacteristic as that of the gauge resistors for correcting said drivevoltage correspondingly to temperature variation of the gauge resistorsso as to cancel the effect of the temperature change of the gaugeresistors of the bridge circuit on the amplification factor of theamplification circuit.
 2. A semiconductor-type pressure transduceraccording to claim 1, wherein said amplification circuit is anoperational amplifier including input and feedback resistors and itsamplification factor is a function of the ratio of the input to feedbackresistance.
 3. A semiconductor-type pressure transducer comprising abridge circuit including a plurality of gauge resistors at least one ofwhich has a resistance changing with an external force, an amplificationcircuit for amplifying the output of said bridge circuit, a drive powercircuit for supplying a drive voltage to said bridge circuit, andamplification compensation means for cancelling the effect of thetemperature change of the gauge resistors of the bridge circuit on theamplification factor of the amplification circuit, wherein said drivepower circuit includes a current source for producing an output currentproportional to the temperature, and a resistor connected to saidcurrent source to pass the output current of said current source, thevoltage drop across said resistor being used as a drive power source forthe bridge, said resistor having the same temperature characteristics asthat of said gauge resistors.
 4. A semiconductor-type pressuretransducer, comprising a bridge circuit including a plurality of gaugeresistors at least one of which has a resistance which varies with anexternal force, an amplification circuit connected to an output of saidbridge circuit for amplifying the output of said bridge circuit, and adrive power circuit connected to a power source terminal of said bridgecircuit connected to a power source terminal of said bridge circuit forsupplying a drive voltage to said bridge circuit, wherein said drivepower circuit includes means for correcting said drive voltagecorrespondingly to temperature variation of the gauge resistors so as tocancel the effect of the temperature change of the gauge resistors ofthe bridge circuit on the amplification factor of the amplificationcircuit, wherein said drive power circuit includes a current source forproducing an output current proportional to the temperature and saiddrive voltage correcting means comprises a resistor connected to saiddrive power circuit to pass therethrough the output current of saidcurrent source and having a temperature-resistance characteristicsubstantially the same as that of the gauge resistors, the voltage dropacross said resistor being used as a drive power source for producingthe drive voltage to be supplied to said bridge circuit.
 5. Asemiconductor-type pressure transducer comprising a bridge circuitincluding a plurality of gauge resistors at least one of which has aresistance variable with an external force, an amplification circuitconnected to an output of said bridge circuit for amplifying the outputof said bridge circuit, a drive power circuit connected to a powersource terminal of said bridge circuit for supplying a drive voltage tosaid bridge circuit, and resistor means connected between the output ofsaid bridge circuit and said amplification circuit and having the sametemperature characteristic as that of the gauge resistors so as tocancel the effect of the temperature variation of the gauge resistors onthe amplification factor of the amplification circuit.
 6. Asemiconductor-type pressure transducer according to claim 5, whereinsaid amplification circuit is an operational amplifier including inputand feedback resistors and its amplification factor is a function of theratio of the input to feedback resistance.